Response of the Southern Ocean Overturning Circulation to Extreme Southern Annular Mode Conditions. Issue 22 (11th November 2020) - Stewart, K. D.
Abstract: The positive trend of the Southern Annular Mode (SAM) will impact the Southern Ocean's role in Earth's climate; however, the details of the Southern Ocean's response remain uncertain. We introduce a methodology to examine the influence of SAM on the Southern Ocean and apply this method to a global ocean‐sea ice model run at three resolutions (1°, (1/4)°, and (1/10)°). Our methodology drives perturbation simulations with realistic atmospheric forcing of extreme SAM conditions. The thermal response agrees with previous studies; positive SAM perturbations warm the upper ocean north of the wind speed maximum and cool it to the south, with the opposite response for negative SAM. The overturning circulation exhibits a rapid response that increases/decreases for positive/negative SAM perturbations and is insensitive to model resolution. The longer‐term adjustment of the overturning circulation, however, depends on the representation of eddies, and is faster at higher resolutions. Plain Language Summary: The Southern Ocean has accounted for the vast majority of the global ocean heat uptake since the early 2000s. The atmospheric winds over the Southern Ocean play a leading role in its ability to uptake heat, by way of driving much of the Southern Ocean circulation. Observations of these winds indicate that they have been steadily changing over the past few decades, and hence, so too is the Southern Ocean heat uptake. However, despite recent research efforts, the details of the Southern Ocean's response to these changing winds remain uncertain. Here we introduce a novel methodology to examine the Southern Ocean's response to changing winds. We perform numerical simulations with a global ocean‐sea ice model suite that spans a hierarchy of spatial resolutions and driven by realistic atmospheric forcing conditions. The initial response of the Southern Ocean circulation to changes in winds is robust across the model suite and insensitive to model resolution; longer‐term response, however, depends on the representation of eddies in the model. Key Points: New methodology to examine the Southern Ocean response to climate extremes using perturbation simulations with realistic atmospheric forcing Southern Ocean overturning circulation exhibits a rapid response that is sustained in depth‐latitude space but decays in density‐latitude space Adjustment of the ocean interior occurs along isopycnals at a rate of order 1° latitude per year, depending of the representation of eddies